According to U.S. Pat. No. 6,077,491, the disclosure of which is expressly incorporated by reference, an aqueous solution of urea is converted to a gaseous product stream of ammonia and carbon dioxide at a rate which is essentially matched to the amount of ammonia required for the removal of nitrogen oxides from combustion gas streams by the SCR or SNCR NOx control methods. This process generally conforms to the following general operating conditions.
An aqueous solution of urea or mixtures of urea and urea precursors having a concentration of about 1% to about 76% by weight of solids is fed into a reactor. The urea therein is hydrolyzed at temperatures of at least 110xc2x0 C. up to about 300xc2x0 C. and under pressures of about 20-500 psig, the temperature or pressure of the reaction mixture being controlled by the input of heat to produce a gaseous product stream of ammonia, carbon dioxide and water. The heat input is maintained at a level rate sufficient to meet the demands for ammonia which is equal to that required to reduce essentially all of the nitrogen oxides present in the combustion gas stream.
It is essential that the amount of ammonia injected into the combustion gas stream be carefully balanced to the amount required to scrub out the nitrogen oxides. If excess ammonia is injected, it can be discharged from the combustion gas stack, creating a hazard. The problem of ammonia slip has been recognized, for example, in Bowers U.S. Pat. No. 4,719,092, Fellows U.S. Pat. No. 5,098,680 and Spokoyny U.S. Pat. No. 5,237,939. However, none of these patents relate to the scrubbing of nitrogen oxides (NOx) from stack gas by the injection of ammonia gas continuously generated from an aqueous urea solution.
According to the above referred to patent, the hydrolysis reactor pressure is controlled by the heat input to a hydrolysis reactor and the gas takeoff rate is controlled by an adjustable control valve, which adjusts to match the required amount for removal of the nitrogen oxides in the combustion gas streams. Emergency pressure relief can be on either the gas side or the liquid side of the reactor. In both cases, a vent can be provided which is connected to a water containing dump tank which serves to trap ammonia gas and prevent its release to the atmosphere. Cool water in the dump tank serves to stop the hydrolysis process and prevent further generation of ammonia.
This invention comprises an improved process to provide a pressurized gas stream useful for removing nitrogen oxides from a combustion gas stream which includes the steps of
(a) hydrolyzing urea in aqueous solution in a closed reactor to evolve gaseous ammonia at a rate essentially balanced to the amount required to remove nitrogen oxides from the combustion gas stream; and
(b) contacting said gaseous ammonia with said combustion gas stream;
the improvement wherein the pressure in the reactor is maintained within a pre-selected range when the demand for ammonia required for nitrogen oxide removal suddenly drops by cooling the solution within the hydrolysis reactor by heat exchange either within or external to the reactor in response to rapid changes in demand for ammonia required to remove said nitrogen oxides.
This invention relates to an improved process to provide a pressurized gas stream useful for removing nitrogen oxides from a combustion gas stream by SNCR (Selective Non-Catalytic Reduction), or SCR (Selective Catalytic Reduction), which comprises the steps of:
a) feeding an aqueous solution of urea or mixtures of urea containing biuret and/or ammonium carbamate, having a concentration of about 1% to about 76% by weight of solids into a reactor and hydrolyzing the urea therein at temperatures of at least 110xc2x0 C. up to about 300xc2x0 C. and under pressures of about 20-500 psig, the temperature or pressure of the reaction mixture normally being controlled by the input of heat to the reactor to produce a gaseous product stream of ammonia, carbon dioxide and water at a rate sufficient for external use in step d), and a residual liquid phase reaction medium containing unreacted urea, biuret and/or ammonium carbamate;
b) separating the gaseous product stream at a controlled pressure and flow rate;
c) retaining the liquid phase reaction medium in the reactor for further conversion to gaseous ammonia and carbon dioxide, and/or recycling at least a portion of the reaction medium back into the reactor, a urea dissolver, or the feed solution to the reactor for further conversion; and
d) withdrawing the gaseous ammonia and carbon dioxide-containing product stream and feeding it for external use at a controlled rate which is approximately the amount necessary for the demands of said external use in removing said nitrogen oxides;
the improvement wherein the pressure in the reactor is maintained within the recited range when the demand for ammonia for said external use suddenly drops by cooling the solution within the hydrolysis reactor by heat exchange either within or external to the reactor in response to rapid changes in demand for ammonia required to remove said nitrogen oxides.
In the process of this invention, an aqueous solution of urea is converted to a gaseous product stream of ammonia and carbon dioxide for use in the removal of nitrogen oxides from combustion gas streams by the SCR or SNCR NOx control methods. The solution of urea is provided at a specific concentration and is pumped into a hydrolysis reactor at a controlled rate. The reaction is endothermic and heat is required. The heat input to the reactor is controlled to maintain a constant gas pressure in the reactor. In the reaction, the urea first hydrolyzes to ammonium carbamate from which the gaseous ammonia-carbon dioxide product mixture is formed and then fed at a controlled rate to the distribution grid in the combustion gas duct. For some applications, the gas mixture is diluted with air, steam or combustion gas to improve mixing and contact with the nitrogen oxides in the combustion gas stream. The gas flow is adjusted to match the ammonia rate with the NOx in the combustion gas stream.
The reaction in the reactor is:
(x)H2O+NH2CONH2xe2x86x922NH3+CO2+(xxe2x88x921)H2O
The rate of generation of ammonia by this reaction is given by the Arrenhius equation. Thus the rate of ammonia generation =Aexe2x88x92b/kT where A is proportional to the number of moles of water and urea and b is the free energy for the reaction. Controlling the amount of urea solution in the reactor and the temperature of the reactor controls the generation rate of ammonia. Below about 110xc2x0 C., the reaction does not occur unless a catalyst is used. The invention contemplates the use of catalysts which allow temperatures below 110xc2x0 C. In the process, the temperature is self-regulated by controlling the heat input into the reactor to maintain a constant pressure. The rate of increase in ammonia generation can be rapid since it is only necessary to provide access heat input to increase the temperature of the reactor. Increasing the reactor temperature from 140xc2x0 C. to 158xc2x0 C. increases the ammonia generation by 300%.
The rate of NOx reduction with ammonia produced from urea is the same as with neat ammonia. The SAR process equations are:
Removal of NOx by Ammonia:
4NO+4NH3+O2xe2x86x924N2+6H2O
2NO2+4NH3+O2xe2x86x923N2+6H2O
Removal of NOx by Urea:
4NO+2CO(NH2)2+O2xe2x86x924N2+4H2O+2CO2
2NO2+2CO(NH2)2+O2xe2x86x923N2+4H2O+2CO,
The urea solution can be used in the concentration range of about 1 to 76%, and preferably is operated at a 40% or 50% urea concentration, which is fed to the reactor with a positive displacement type pump. The urea solution is pumped to the hydrolysis reactor at a controlled rate to maintain a constant level in the reactor. The reactor liquid level is monitored with a differential pressure transmitter/controller, which controls the reactor feed rate, to maintain a constant liquid level in the reactor. The feed rate is controlled either with a proportioning pump or by controlling the rate of take off from a constant speed pump equipped with a re-circulation line.
The generation of ammonia at a controlled rate by hydrolysis of urea has been developed for the control of NOx emissions with SNCR and SCR system. In the process heat is applied to drive the hydrolysis reaction. In normal operations the demand for heat input is adjusted to meet production requirements. One means to control the heat input is to monitor the pressure in the reaction vessel and adjust the heat to maintain a constant gas pressure as disclosed in the above-identified patent application. As additional ammonia is required, heat is added. As the ammonia requirement decreases, the heat addition is reduced and the heat stored in the reactor solution and reactor vessel is used up as the temperature of the solution decreases to meet the new demand requirement. However, in the case of a sudden or instantaneous decrease in the demand for ammonia, such as occurs when the amount of oil, coal or gas being charged to the boiler of a power plant is rapidly cut back, the heat stored in the hot reactor solution and in the reactor vessel will continue to generate ammonia, excessively increasing the pressure in the reactor vessel until the temperature of the solution is lowered to below the generation point for ammonia and the hydrolysis reaction stops. To limit this pressure build-up, solution or gas must be discharged from the vessel unless the temperature of the solution and vessel are reduced. However, withdrawal of hot solution or gas presents major problems which this invention avoids.